BP's partners hit back at report which says no single factor caused the April explosion at the Deepwater Horizon rig.

Millions of gallons of oil spilled into the Gulf of Mexico after the explosion on the drilling platform [AFP]

The energy firm BP and its Gulf of Mexico oil well partners have traded blame over the explosion that led to the spill of 4.9 million barrels of oil into the sea.

While accepting some responsibility for the April blast at the Deepwater Horizon rig, BP has in a report pointed the finger at what it said were major failures by Transocean Ltd, the operator of the rig, and Halliburton, which cemented the deep-sea well.

But Transocean said on Wednesday that BP was seeking to conceal the key factor that led to the rig explosion - the "fatally flawed" well design.

"In both its design and construction, BP made a series of cost-saving decisions that increased risk," Transocean said.

Halliburton also rejected the findings of the report, saying it contained "substantial omissions andinaccuracies" and stressed it was fully indemnified for any allegations in the document.

The April 20 explosion left 11 people dead and efforts to stop the massive oil spill took several months to succeeded.

The BP report, published on Wednesday after an internal investigation, said no single factor caused the oil spill and that multiple companies and individual work teams contributed to the "accident".

'Multiple errors'

It said the accident rose from "a complex and interlinked series of mechanical failures, human judgements [and] engineering design".

BP stopped short of admitting gross negligence, but proposed 25 recommendations designed to prevent a repetition of the explosion.

"It is evident that a series of complex events, rather than a single mistake or failure, led to the tragedy," Tony Hayward, BP's outgoing chief executive, said

"Multiple parties, including BP, Halliburton and Transocean, were involved."

Hayward said failings included a "bad cement job" at the bottom of the well that allowed gas and liquids to flow up the production casing.

The Gulf Restoration Network, an environmental organisation that seeks to protect the natural resources of the Gulf region, rejected the outcome of the internal investigation.

"Instead of blaming their own decision not to use more sensors on the well, they blamed the concrete," Aaron Viles, campaign director for the Gulf Restoration Network, told Al Jazeera.

"They are positioning themselves to have another tool as they seek to share responsibility, and ultimately liability, with Transocean and Halliburton," he said, adding that "it is pretty transparent, similar to my three year old daughter when she tells me it is not time for bed yet.

"When they said it was a succession of mistakes that led to this blow-out, … I think that is true. But I do think that they put themselves in a pretty good light as you work through this report."

Costly clean-up

BP denied it was shifting blame and accepted that one of its representatives, in conjunction with Transocean, had incorrectly interpreted a safety test that should have flagged up risks of a blowout.

The four-month probe, led by Mark Bly, BP's head of safety and operations, is viewed as key to how the company will defend itself against legal proceedings involving the spill.

The company said last week it had spent $8bn so far responding to the spill, but analysts expect the final bill to run to billions of dollars.

The leaking Macondo well has now been secured but the disaster is being examined in a string of court cases and probes, including a criminal investigation being carried out by the US justice department.

US politicians have accused the oil giant of sacrificing safety to improve its profit margin - a charge Hayward denied during a session in congress in June.

The US government is currently conducting its own investigation into the oil disaster.